Studies are continuing on the process of recognition of foreign antigen by human T cells, particularly with respect to the role of T cell surface molecules in cytotoxic T cell (CTL) interaction. Recognition of the DP antigens has been used as a model system using a panel of DPw2-specific CTL clones. Visible conjugate formation between effector and target has been analyzed by flow cytometry using a new dye combination we have identified which avoids covalent modification of the cell surface. In our system there is strong antigen-nonspecific conjugate formation and the specificity of killing is determined at the level of triggering. We believe the antigen-nonspecific conjugates to be a useful model system in which to understand the role of monomorphic cell surface molecules (e.g., LFA-1, LFA-2, LFA-3) in interactions of lymphoid cells. Studies of the T3 molecule have confirmed its unique role in triggering T cells. Anti-T3 immobilized on assay plates show a unique enhancement of their ability to inhibit CML; and anti-T3 expressing hybridomas trigger their own lysis by DPw2-specific CTL. Surprisingly, anti-LFA-1 inhibits anti-T3 triggered lysis, suggesting that LFA-1 plays a crucial role even in this xenogeneic interaction which bypasses antigen-specific steps.